Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps--non-directional and for any polarization--to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.

Abstract:Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localised plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare. (2001)

Mesostructured metallic substrates composed of square pyramidal pits are shown to confine localized plasmons. Plasmon frequency tuning is demonstrated using white light reflection spectroscopy with a wide range of structure dimensions from 400 to 3000 nm. Using a simple plasmon cavity model, we demonstrate how the individual pit morphology and not their periodicity controls the resonance frequencies. By measuring the surface-enhanced Raman scattering ͑SERS͒ signals from monolayers of benzenethiol on the same range of mesostructures, we extract a quantitative connection between absorption, field enhancement, and SERS signals. The match between theory and experiment enables effective design of plasmon devices tailored for particular applications, such as optimizing SERS substrates. DOI: 10.1103/PhysRevB.76.035426 PACS number͑s͒: 73.20.Mf, 42.70.Qs, 72.15.Rn, 81.07.Ϫb Raman scattering is a crucial spectroscopic technique for identifying molecules through their vibrational resonances and has increasingly important applications in monitoring low concentrations of impurities or trace biomolecules.1-3 It has also been suggested for direct monitoring of coupling of molecular distortions and electronic transport in molecular electronics. [4][5][6] However, the terribly weak Raman cross section has always made such application problematic. The enormous enhancement in cross section when the molecules are held close to a metal surface with nanoscale roughness 7,8 has driven the hope that surface-enhanced Raman scattering ͑SERS͒ will become a viable and reproducible diagnostic. While improvements have been made in terms of enhancement factor, reproducibility, and in understanding that plasmons underpin such enhancements, 9 it is unclear how to precisely design nanostructures to optimize the Raman signatures. Over the last five years, we have shown that mesostructured metal films comprised of arrays of voids form excellent surfaces for localizing plasmons while retaining strong coupling to external light.10-13 Recently, we showed, using angularly resolved SERS on such void substrates, that incident photons are transducted both into and out of molecules via plasmons, 14 giving hope that reproducible substrates can be designed for specific applications. While most SERS research has focused on using the nanoscale junctions between metallic particles such as colloids 15 or lithographic arrays 16 that have broad plasmon resonances which can only be tuned through control of shape anisotropy or gap dimensions, the voids show strong sharp tunable plasmon resonances. Previous work has shown that optimized samples possess plasmon absorption that lies between the laser wavelength and outscattered Raman emission; 17-19 however, they are unable to make clear the quantitative link between plasmon resonant absorption and SERS emission.In this paper, we present calibrated spectroscopic measurements on systematically engineered plasmonic mesostructured metal surfaces. We show the quantitative connection between the resonance arising from ...

There have been numerous efforts to increase the efficiency of solid-state lighting, lightemitting diodes (LEDs) and displays during the last decades. [1][2][3] As the technologies for fabricating GaN-based LEDs and for synthesizing semiconductor colloidal nanocrystals (NQDs) mature, hybrid NQD-GaN LEDs are becoming promising candidates for highly efficient multi-color lighting. The high quantum yield and photostability of colloidal NQDs offer the possibility for flexible, low cost, large area, and simply-processed optoelectronic devices, while their emission color can be tuned from the visible to the near infrared range by either changing their size or chemical composition. [4] Also the epitaxial growth of GaN has now reached the stage where GaN-based LEDs have an internal quantum efficiency of 80%. [5] Although their external quantum efficiency is inevitably limited by total internal reflection due to the high refractive index contrast with air, several approaches to improve the outcoupling efficiency have been realised implementing smart photonic crystal and waveguide designs. [6,7] Color conversion LEDs consisting of colloidal NQD emitters pumped by GaNbased LEDs overcome the drawback of NQDs, i.e. low carrier transfer. [8] A thin NQD layer deposited on LED surface absorbs the high energy photons that are electrically generated in the LED and subsequently reemits lower energy photons. As a result, there is no charge transfer among colloidal NQDs involved in this color conversion process. However, the efficiency of radiative energy transfer is relatively low, <10%, due to several energy loss steps in the transfer process, i.e. waveguided leaky mode losses, light scattering from the NQDs and

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